The present invention relates generally to gun fired projectiles and is more particularly concerned with a new and improved gun fired projectile having reduced base drag.
It has been known for a long time that a gun fired projectile, such as a bullet, shell, etc., has a tendency during flight of forming a partial vacuum condition adjacent the flat, rear or base of the projectile. The partial vacuum or low pressure condition at the base of the projectile provides, in effect, a force component acting against the direction of motion of the projectile. This force component is commonly referred to as "base drag." In fact, at the transonic or low supersonic speeds of conventional gun fired projectiles, for example, at speeds of about Mach 2, the base drag may constitute more than 50 percent of the total drag acting on the projectile and reducing its flight velocity.
Various pyrotechnic materials have been loaded into base cavities in the projectiles and ignited at the time of discharge so that the burning of the pyrotechnic at least partially fills the vacuum generated by the projectile, thus reducing the base drag and aiding the flight of the projectile. The projectiles containing such pyrotechnic compositions are commonly referred to as "fumers." Some presently available fumers use dry pyrotechnic powder components that must be formed into a consolidated state by mixing with binding agents to hold the pyrotechnic charge in the projectile cavity during flight. However, such mixes are frequently poor performers relative to base drag reduction primarily due to the presence of the binding agents. Other fumers of the castable type also are undesirable because of their poor retention strength and inconsistent burning rate and performance.
Recently it was suggested in Puchalski, U.S. Pat. No. 3,886,009, that the pyrotechnic mixture used to reduce base drag should burn at high yield temperatures of at least 2700.degree. C. and preferably 3500.degree. C. in order to insure the production of sufficient pressure within the base area during flight. Pyrotechnic mixtures of the high temperature type mentioned in that patent required stoichiometric proportions of fuel and oxidizer and contained about 30 percent magnesium powder, 50 percent strontium nitrate, 8 percent calcium resinate and 10 percent gelatin. The patent stated that use of color intensifiers, such as polyvinyl chloride, in the pyrotechnic composition produced a deleterious effect on the base drag characteristics of the projectile.
In accordance with the present invention it has been found that reduced projectile base drag can be achieved without the high yield temperatures mentioned hereinbefore by utilizing a pyrotechnic that exhibits the characteristics of rapid burning in the near wake area of the projectile, i.e. immediately adjacent the flat base, and the production of fuel-rich combustion products that permit secondary reactions with the atmosphere in the near wake thereby improving specific impulse of the fumer. Accordingly, it is an object of the present invention to provide a fumer loaded with a pyrotechnic composition having these characteristics.
Another object of the present invention is to provide a new and improved fumer of the type described that exhibits high performance and a consistent burning rate over a prolonged period thereby improving the total impulse of the fumer and providing sustained base drag reduction.
Still another object of the present invention is to provide a new and improved fumer of the type described that utilizes a pyrotechnic composition of high adaptability since it is castable, moldable and pressable yet does not exhibit the low performance characteristics previously associated with such materials. Included in this object is the provision for a versatile composition that is easily adapted to existing projectile configurations while at the same time allows latitude in the design of new projectiles.
Yet another object of the present invention is to provide a new and improved fumer of the type described that utilizes a pyrotechnic composition having greater than stoichiometric amounts of fuel in the fuel-oxidizer mix and provides fuel-rich combustion products. Included in this object is a provision for a pyrotechnic fumer that also includes about 20 percent by weight and more of a plasticized resin color intensifier in conjunction with an active oxidizer, such as ammonium perchlorate and the like, for the resin.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are accomplished in accordance with the present invention by providing a fumer comprising a projectile having a base cavity and a pyrotechnic charge positioned within that cavity. The pyrotechnic charge is of the metal powder/alkaline earth metal compound type and contains about 20 percent by weight and more of a plasticized resin and about 10 percent by weight and more of an active oxidizer of the perchlorate type. The metal powder is present in greater than stoichiometric amounts relative to the alkaline earth metal compound and the charge provides rapid burning in the near wake area and fuel-rich combustion products.
A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description which sets forth an illustrative embodiment and is indicative of the way in which the principles of the present invention are employed.